Refractory linings employed in basic oxygen process steel-making furnaces must have sufficient strength to support the charge of molten metal in the vessel. In addition, the lining must be resistant to erosion or abrasion caused by the molten metal and molten slag. The porosity of the coked refractory composition is important with respect to the useful life of the lining in the vessel; low porosity refractory linings are relatively impervious and thus the molten metal or molten slag do not readily penetrate the surface of the lining and erode the refractory.
In order to withstand the slagging conditions found in present-day basic oxygen steel furnaces, refractory linings must have a porosity after coking of less than 15% and preferably less than 13% (as determined by ASTM procedure C-607). It is also essential that the refractory shapes used for such linings remain strong from room temperature up to a temperature at which the carbonaceous bonding material begins to coke (approximately 850.degree.-950.degree. F). If a weakness occurs as a temperature below the temperature at which the coke begins to form, a soft zone will be created in the lining, resulting in spalling during initial heat-up of the vessel. After coking, the refractory shapes to be useful in basic oxygen furnaces, must have transverse strength of at least 500 psi.
The conventional method of producing shaped refractory articles for use as linings in basic oxygen blowing steelmaking vessels involves hot forming of pitch bonded refractory bricks. However, the hot forming process has many disadvantages, and the need has existed for a technique for producing pitch bonded refractory shapes which can be molded at room temperatures. In copending application Ser. No. 413,250, filed Nov. 6, 1973, by the present inventors, there is disclosed a pitch-containing binder composition and a method of preparing refractory shapes at room temperature utilizing the disclosed binder composition. As taught in said copending application Ser. No. 413,250, the entire disclosure of which is hereby expressly incorporated by reference, refractory shapes bonded with the binder composition disclosed therein have a porosity after coking of less than 15% and maintain high strength through all temperature ranges up to the operating temperature of basic oxygen process steel furnaces.
While the compositions and processes disclosed in Ser. No. 413,250 represent significant improvements in the art, it has now been found that further improvements and advantages can unexpectedly be realized if the cold pitch bonded shaped refractory articles made in accordance with the teachings of application Ser. No. 413,250 are subjected to impregnation with a second binder composition, preferably a hot liquid pitch-containing binder composition. It has been surprisingly found that the use of a second bonding stage advantageously results in reduced porosity and increased density after coking, increased strength through all temperature ranges including after coking, and increased carbon after coking. All of these improvements increase the life expectancy of furnace linings made using the shaped refractory articles manufactured in accordance with the present invention, even as compared to that attainable by the cold pitch bonded shaped refractory articles without the second stage impregnation.